docs: deprecate RISC-V default machine option

Commit 5b4beba124 ("RISC-V Spike Machines") added the Spike machine and
made it default for qemu-system-riscv32/64. It was the first RISC-V
machine added in QEMU so setting it as default was sensible.

Today we have 7 riscv64 and 6 riscv32 machines and having 'spike' as
default machine is not intuitive. For example, [1] is a bug that was
opened with the 'virt' board in mind, but given that the user didn't
pass a '-machine' option, the user was using 'spike' without knowing.

Being explicit in the command line is desirable when we have a handful
of boards available, so deprecate the default machine setting from
RISC-V.

[1] https://gitlab.com/qemu-project/qemu/-/issues/2467

Suggested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20250404122858.241598-1-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

docs/about/deprecated.rst

@@ -304,6 +304,23 @@ online to check that this board did not completely bitrot yet). It is
 recommended to use another MIPS machine for future MIPS code development
 instead.
 
+RISC-V default machine option (since 10.0)
+''''''''''''''''''''''''''''''''''''''''''
+
+RISC-V defines ``spike`` as the default machine if no machine option is
+given in the command line.  This happens because ``spike`` is the first
+RISC-V machine implemented in QEMU and setting it as default was
+convenient at that time.  Now we have 7 riscv64 and 6 riscv32 machines
+and having ``spike`` as a default is no longer justified.  This default
+will also promote situations where users think they're running ``virt``
+(the most used RISC-V machine type in 10.0) when in fact they're
+running ``spike``.
+
+Removing the default machine option forces users to always set the machine
+they want to use and avoids confusion.  Existing users of the ``spike``
+machine must ensure that they're setting the ``spike`` machine in the
+command line (``-M spike``).
+
 
 Backend options
 ---------------